I heard that a wavefunction applies to a quantum system.
But what is a quantum system? I am new to quantum mechanics, sorry for asking a basic question.
The shortest answer is that a quantum system is any system that obeys the laws of quantum mechanics. This means that:
This explanation probably gives rise to even more questions - "what is a complex vector space ?", "what is a linear operator ?", "what is an eigenstate ?" etc. - which are best answered by reading an introduction to quantum mechanics such as this one at Wikipedia